Development of wireless communication technology has enabled transmission/reception and sharing of data between different electronic apparatuses. For example, it is possible to directly transmit multimedia files, including image files, stored in a digital camera or a multimedia playback device, to a smartphone or a laptop computer. Technology for transmitting data between electronic apparatuses is also useful in medical fields. For example, information regarding patients obtained from a medical electronic apparatus, such as an electrocardiography (ECG) sensor, which is attached to a human body, can be transmitted to a personal computer, a mobile communication terminal, etc. Such wireless transmission/reception is largely based on Wi-Fi technology, with which a limited number of electronic apparatuses (e.g. laptops, smartphones, etc.) have been equipped. Recently, more diversified apparatuses (e.g. game consoles, printers, TVs, etc.) have also been equipped with Wi-Fi technology.
An electronic apparatus that is designed to be portable or an apparatus that is designed to be attached to a human body needs an antenna device that can exhibit stable performance with a compact size, in order to reduce discomfort experienced by the user or patient. An electronic apparatus using a metallic case, such as a small digital camera, has a pleasing aesthetic appearance, but the material of the case makes it difficult to secure stable antenna performance. Therefore, part of the metallic case may be removed to secure stability of wireless transmission/reception. However, removal of a part of the case compromises the aesthetic appearance provided by the metallic case. A medical electronic apparatus to be attached to a human body preferably gives the patient, to whom it is attached, no discomfort, while the antenna device exhibits stable operating characteristics. A bowtie dipole antenna having a size of 40 mm×25 mm, which has been commercialized as an ECG sensor antenna, exhibits a radiation efficiency of 95% before being attached to a human body, but the radiation efficiency drops to 5% in an actual environment of use, e.g. when attached to a human body.
The above-mentioned antenna devices are capable of wireless transmission/reception in poor operating environments, e.g. when installed inside a metallic case or when attached to a human body, as long as they are manufactured with a sufficient size. However, as described above, a compact electronic apparatus or an apparatus that is supposed to be attached to a human body needs to be compact and light, while securing stable operating characteristics of the antenna device, in order to make the user less uncomfortable.
Further to the above considerations, the radiator of an antenna device requires a distance of at least ¼ of the signal wavelength from the ground surface of an electric conductor. If the distance between the radiator and the ground surface of the electric conductor is less than ¼ of the signal wavelength, a surface current is induced on the ground surface in the opposite direction of the current flowing through the radiator. In that case, the signal current of the radiator is offset by the current on the ground surface, thus preventing the antenna device from functioning. The magnetic conductor operates as a component having the function of an open circuit with a considerably high resistance at a specific frequency. This can be realized by periodically arranging a cell pattern of an intended specific unit on the electric conductor, and a magnetic conductor made in this manner is referred to as an artificial magnetic conductor (AMC). A radiator arranged on the AMC may be arranged closer to the ground surface than the radiator of a conventional antenna device. However, there is a limit to making an AMC, which requires periodic arrangement of a cell pattern of a specific unit, small enough to be applied to a compact electronic apparatus and an apparatus to be attached to human bodies.
Antenna devices using AMCs are disclosed in a paper published by IEEE ICICS in 2011, entitled “A Wideband High Gain Dipole EBG Reflector Antenna (P. Lau, etc.)”, in a paper published at the Antenna and Propagation Conference in 2009, entitled “Ultra Thin Dipole Antenna backed by new planar artificial magnetic conductor (M. Al-Nuaimi etc.)”, etc.
Antenna devices disclosed in the above-referenced papers utilize AMCs composed of a unit cell combination with a 7×7 arrangement or a unit cell combination with a 9×5 arrangement, and their horizontal×vertical size exceeds 50 mm×50 mm, placing a limit on mounting them on compact electronic apparatuses or medical electronic apparatuses which are attached to human bodies.
Accordingly, there is a need for an improved antenna device that is compact and light, and an electronic apparatus having the same.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.